(57) An optical fibre structure usable in amplifying optical signals, comprising a length
of optical fibre (1) which includes two uniformally spaced monomode optical cores
(2,4) located within a common cladding (6) so as to provide two optical guidance paths,
the optical characteristics of at least the cores being different such as to give
the two guidance paths different propagation constants the values of which coincide
for a predetermined coupling wavelength, one of the cores including a fluorescent
material capable of producing stimulated emission. There is also disclosed a method
of amplifying an optical signal using such an optical fibre structure, comprising
arranging for the wavelength (λs) of the optical signal, the fluorescent wavelength of said material in said one core,
and said predetermined coupling wavelength all to be substantially the same, launching
optical pump energy having a different wavelength (λp) into said one core to pump the fluorescent material, launching the optical signal
into the other core so that the optical signal transfers repeatedly between the two
cores due to optical coupling and, when in said one core, gives rise to stimulated
emission substantially at its own wavelength from the fluorescent material thereby
becoming amplified, and extracting the amplified optical signal from the fibre structure
when it is in said other core. A device (Figure 4) using this method is also disclosed.
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